This is a true story. Geoff "Buff" Hoon (say it out loud) pretends to be David Cameron when Gordon Brown rehearses for Prime Minister's Questions.

Buff - chief whip in charge of government discipline - trots obediently round to Number Ten at least once a week to play leader of the opposition to Big Gordie's premier.

Not surprisingly, this little play has set tongues wagging at Westminster. I mean, you have to be scraping the bottom of the barrel to cast Hoon as the PM-in-waiting.

Even less credible, I hear Buff, whose only previous claim to fame was shaving off his weedy tache, credits himself with improving Brown's performance.

Considering how wooden Brown is at the Despatch Box, that would not be hard.

But I suspect the Prime Minister is getting better because friends have told him to cool it and not to lose his temper with Dave the Dude.

All of which begs the question: who pretends to be Prime Minister when Cameron rehearses for the weekly ordeal? There is no shortage of failed Tory leaders on the back benches - Iain Duncan Smith, William Hague, Michael Howard.